This invention relates to an off-air phase lock technique for cable television systems which use modulators for cable channels and, more particularly, relates to an apparatus, system and method for providing off-air phase lock capability that allows locking the frequency and phase of a CATV modulator output signal to that of an off-air signal.
Cable television systems may transmit television signals at the same frequencies as broadcast television signals. Accordingly, it is possible, if not probable, that a television receiver will pick up the broadcast television signal (i.e. the off-air signal) directly and will also receive the transmitted channel over the cable system. The signals at the same frequency may carry the same material (e.g., broadcast channel 7 will transmit the same information as cable transmitted channel 7). If television stations are carried at the same frequency on cable as broadcast, the off-air broadcast signals may interfere with the cable signals, causing lines and beats to appear in the picture. Phase-locking the two signals reduces the effects of interference.
In a cable television xe2x80x9cprocessorxe2x80x9d, a broadcast television channel may be converted to an intermediate frequency for controlling its level and frequency response. The signal is then typically converted back up, possibly to a different channel, for transmission on a cable television system. In this type of conventional system, it is not difficult to provide phase lock between the input and output frequencies. This occurs when the broadcast and cable channels are at the same frequency, and the solution is to use the same local oscillator for both conversions, as shown in FIG. 1. More specifically, in FIG. 1, the broadcast television channel signal is received by a first converter (mixer) 10. Local oscillator 30 generates a signal for mixer 10 which mixes with the off-air signal so that the off-air signal is converted to an intermediate frequency (IF) signal and is provided to the cable television processor. The signal is then received by a second converter (mixer) 20 which converts the intermediate frequency signal back up to an RF frequency for transmission. Since both converter (mixer) 10 and converter (mixer) 20 are controlled by the same local oscillator 30, the input off-air signal frequency and the CATV processor output frequency are xe2x80x9cphase lockedxe2x80x9d.
However, the conventional phase lock method described above fails to meet the demands of current cable systems. Present day cable systems now normally use modulators for all cable channels (one modulator per channel), for a variety of performance reasons. The modulator converts standard base band audio and video signals into RF signals, assigns them a channel, and sends them onto the cable distribution system. In this case, such direct control of frequency and phase is not possible, and other means must be found. Present day modulators normally are frequency-agile, capable of producing an output on any cable television channel, and are provided with a reference frequency input. This input is normally used in cases where it is desired to make the outputs of a number of modulators coherent in frequency, for control of distortion products.
Accordingly, it is desired to provide a technique that phase-locks two signals such that the signal on the cable system is at exactly the same frequency as the corresponding off-air signal (i.e., to ensure that the two signals are coherent, i.e. at the same frequency and phase). To provide the coherent feature, the two signals must be phase-locked, thereby ensuring that they are at the same frequency and mining the effects of interference.
The present invention provides an off-air phase lock technique that allows locking both the frequency and phase of a CATV modulator signal to that of an off-air signal so as to reduce interference between the two signals.
In accordance with one embodiment of the invention, the phase lock apparatus accomplishes the frequency and phase signal lock by using a tuner, for receiving the off-air signal and generating an intermediate frequency signal, and a phase-frequency detector, for comparing the frequency and phase of the signal generated by the tuner with the frequency and phase of a signal generated by the modulator based on a reference input signal. The output of the phase-frequency detector is then used to control the reference input signal into the modulator and the reference input signal into the tuner, such that the frequency and phase of the modulator RF output signal is locked to the frequency and phase of the received off-air signal.
In a second embodiment of the invention, the phase lock apparatus locks the frequency and phase of the modulator RF output signal to that of the off-air signal using a channel selection filter and a frequency divider. The divider divides the frequency of the off-air signal, from the channel selection filter, into a first common frequency signal, and a phase-frequency detector compares the frequency and phase of the first common frequency signal with the frequency and phase of a second common frequency signal generated from a reference input signal provided to the modulator. The output of the phase-frequency detector is then used to control the reference input signal provided to the modulator, such that the frequency and phase of the modulator RF output signal is locked to the frequency and phase of the received off-air signal.
Finally, in a third embodiment of the invention, the phase lock apparatus locks the frequency and phase of the CATV modulator RF output signal to that of an off-air signal by receiving the off-air signal at a first tuner and generating an intermediate frequency signal, and receiving the CATV modulator RF signal at a second tuner and generating an intermediate frequency signal, the second tuner having a common reference input frequency with the first tuner. A phase-frequency detector is provided for comparing the frequency and phase of the intermediate frequency signal generated by the first tuner with the frequency and phase of the intermediate frequency signal generated by the second tuner, and the output of the detector is used to control the reference input signal into the modulator.
The above-mentioned and other features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.